This invention relates to a method of forming thin film patterns, particularly thin films such as metallic films, in the fabrication of integrated circuits.
Present trends in the formation of vacuum deposited thin metallic film patterns make the use of etching in the presence of etch-resist photoresist layers to provide the selected pattern. This, in effect, involves the traditional photoengraving or photolithographic etching techniques. However, with the continued miniaturization of semiconductor integrated circuits to achieve greater component density and smaller units in large scale integrated circuitry, the art is rapidly approaching a point where such photolithographic etching of deposited film may be impractical for providing the minute resolution required for the fine linework of metallization, particularly at appreciable metal thicknesses in the order of 8000A - 25,000A, in such large scale integrated circuitry.
An alternative method for forming such metallization in large scale integrated circuitry, which is presently under consideration and use in the art, is commonly denoted by the term "expendable mask method," "lift-off method," or "stencil method." The following references are typical of those describing these known types of methods.
1. T. D. Schlaback et al., "Printed and Integrated Circuitry," pp. 352-353, McGraw-Hill, New York, 1963. PA1 2. K. C. Hu, "Expendable Mask: A New Technique for Patterning Evaporated Metal Films," Electron Packaging and Production, October 1967. PA1 3. M. Hatzakis, "Electron Resist for Micro-Circuit and Mask Production," Journal of The Electrochemical Society, Vol. 116, p. 1033, 1969. PA1 4. H. I. Smith et al., "A High-Yield Photolithographic Technique for Surface Wave Devices," Journal of The Electrochemical Society, Volume 118, p. 821, 1971.
U.S. Pat. No. 3,849,136 entitled "Masking of Deposited Thin Films by Use of a Masking Layer-photoresist Composite," filed July 31, 1973, assigned to the assignee of the present application, is directed to a lift-off method and structure for depositing thin films which avoid the "edge-tearing" problem. The method involves the formation of a metallic masking layer over an initial layer of photosensitive material on the substrate. The photosensitive layer is then over-exposed through the openings in the masking layer, after which the exposed portions of the photosensitive layer are removed chemically, e.g., by photoresist development. Because of this over-exposure, the removed photoresist provides a structure wherein the openings in the masking layer are smaller than the openings in the underlying photosensitive layer. As a result, an overhang of the metallic masking layer is provided over openings in the photosensitive layer. Because of this overhang, when thin films, particularly metal films, are deposited over the structure, and the remaining photoresist is removed by standard lift-off techniques, the "edge-tearing" problem is minimized.
Where lateral widths of the thin film lines, e.g., metallic lines, to be deposited are spaced in the order of 0.5 mils or greater, the method of said copending application provides a satisfactory and workable lift-off technique for depositing thin films, particularly thin metallic films, without any "edge-tearing" problems. However, where the lateral widths of the spacing between such deposited lines is narrower, in the order of 0.05 to 0.25 mils, some difficulty may be expected to arise in maintaining complete adhesion of the metallic mask to the underlying photoresist as well as in maintaining adhesion of the deposited thin film metallic lines.
U.S. Pat. No. 3,873,361, entitled "A method of Depositing Thin Film Utilizing a Lift-Off Mask," filed Nov. 29, 1973, assigned to the assignee of the present application, is directed to a lift-off metod utilizing another overhang type structure which avoids the "edge-tearing" problem and is utilizable even with narrow widths of spacing between lines in the order of 0.05 to 0.25 mils with a minimum of problems. The method of said application does not utilize a photosensitive or active photoresist bottom layer, and forms apertures through the bottom layer by sputter etching rather than by photolithographic techniques.